Typical devices and methods for measuring the time interval between two signals include connecting a source of periodic clock pulses to a clock gate. A first signal is used to enable the clock gate and thereby pass clock pulses of known period through the gate. A second signal is used to disable the clock gate and thereby inhibit the passage of clock pulses through the gate. The output is counted and the time interval is proportional to the number of pulses counted.
Disadvantages with this technique are that the shortest time interval which can be resolved is determined by the period of the clock pulses and the reading obtained may have an error corresponding to .+-.1 pulse count.
Additional error is introduced by using traditional direct control gating methods. When the gate opens it may truncate some fraction of a clock pulse. When closing the gate may again truncate a clock pulse. The response of the counter circuitry to a fraction of a clock pulse cannot be reliably determined. Depending on the time relative to the clock period when the time interval occurs, these fractions of clock pulses may be counted as zero, one or two clock pulses. If a number of time intervals are averaged, the average reading is a function of the response of the counter circuitry to fractional pulses which is difficult to control and a potential source of significant error.
This error can be greatly reduced and resolution improved by synchronizing the opening and closing of the clock gate with the periodic clock pulses and taking the average of a number of time interval measurements as disclosed, for example, in U.S. Pat. No. 3,631,343.
Such time interval averaging counters employing a synchronized clock gate produce valid and useful results for a majority of measurements possible. However, if a repetition rate of time intervals to be averaged in synchronous with the clock rate of periodic pulses from the counter's timebase, then typical averaging methods will not improve resolution beyond a .+-.1 pulse count error.
These synchronous rates are given by ##EQU1## where fo is the time base clock frequency; Q, L, and M are positive integers and L, M are co-prime. The worst case occurs when M=L=1 at which time no averaging at all takes place. For other values of M, partial averaging takes place with ever-increasing effectiveness as M increases. These frequencies, together with a small band of frequencies around each of them, are very numerous, often encountered and somewhat cumbersome to detect. A counter in a synchronous condition typically appears to hang up on some value which may be, but is not limited to, a reading that is an integral multiple of the clock period and averaging intervals will not increase the resolution of the measurement.
Similar limitations in resolution are observed in counters which pass a signal to be measured through a clock gate whose time window is determined by a fixed number of pulses produced at the clock rate by the counter's timebase. The gated signal may be, for example, a pulsed radio frequency signal whose frequency is to be determined. By counting the number of periods of the signal gated and dividing this number by the known time interval of the time window, frequency can be obtained within .+-.1 count. In averaging, a number of these known time intervals of time windows are generated and the gated periods are totalized. The average frequency is then the totalized periods gated divided by the sum of all the time intervals generated. If the unknown frequency and the intervals generated by the timebase exhibit a synchronous relationship, the same problem arises as in the time interval averaging case and statistical averaging does not take place. The fundamental problem is the relative coherence between the gating and the gated signal.